It sounds like a fun project, Katrina. Good luck.
For many years I've been interested in drawing up connections between various songs and sometimes they become a "family tree". We've sung theme concerts using this kind of study.
I'm careful about my sources. I began with the Child Ballads and his notes about them.
Perhaps, for a start it might be easier to make up several family trees. Here are some groups of songs that fit well with that idea:
1. The "Died for Love" songs -- (The Butcher's Boy, Careless Love --so many it's scary) I'd say jump in anywhere and branch out, up, down.
2. The Unfortunate Rake.
3. The Cutty Wren.
You will find yourself heading into other countries -- particulary America but that may not matter.
You'll also find more cousins, brothers and sisters than you will grandparents -- if you know what I mean.
Still -- it sounds great.

Cheers, Joy